High energy accelerator magnet structure



July 26, 1966 H. S. GORDON HIGH ENERGY ACCELERATOR MAGNET STRUCTUREFiled Jan. 20, 1964 INVENTOR. HAYDEN S. GORDON ATTORNEY.

United States Patent 3,263,136 HIGH ENERGY ACCELERATOR MAGNET STRUCTUREHayden S. Gordon, Orinda, Calif., assignor '10 the United States ofAmerica as represented by the United States Atomic Energy CommissionFiled Jan. 20, 1964, Ser. No. 339,043 11 Claims. (Cl. 317-200) Theinvention described herein was made in the course of, or under ContractW7405Eng48 with the United States Atomic Energy Commission.

The present invention relates to apparatus for accelerating chargedparticles to high energies and more particularly to the electromagnetsused therein for the purpose of maintaining the particle beam in asubstantially circular orbit. The invention provides a uniqueconfiguration for the magnet core and associated excitation coil whichenables a more intense and broader magnetic field to be established inthe beam region while reducing the amount of magnet material required.

Particle accelerators of the type which recirculate a particle beamaround a curved path require a massive magnet structure to hold the beamin the desired orbit. In higher energy machines of this type particlesare injected into the accelerator with considerable initial energy andthe radius of the beam orbit is held almost constant, while theparticles are gaining energy, by increasing the strength of the magnetfield in the course of each accelerating cycle. Accordingly the magnetis ring-shaped in overall configuration. To facilitate construction ofthe magnet, and to provide a series of relatively unobstructed regionsaround the beam orbit, the magnet is usually comprised of a plurality ofseparate curvilinear sections or short straight sections some of whichare spaced apart and connected by a linear vacuum tubulation throughwhich the beam passes.

To effect optimum focussing of the circulating ion beam, recentaccelerators of this type have frequently included magnets which providean alternating gradient field. In particular, periodic ones of themagnet sections have a magnetic field, approximately normal to the planeof the particle orbit, in which the flux lines bow outwardly withrespect to the center of the accelerator. In intervening ones of themagnet sections the flux lines have an opposite curvature. Any tendencyof component particles to deviate from the preferred orbit is suppressedby passage of the beam through a series of such magnet sections, thetheoretical basis for this strong focussing effect being understoodWithin the art and being described, for example, by E. D. Courant etal., The Physical Review 88, 1190 (1952). This effect concentrates thebeam at the desired orbit and allows a concomitant reduction of thecross-sectional area of the vacuum envelope and magnet structure.

Considering now the structure of an individual magnet section, it hasheretofore been a common practice to make use of what is termed aC-magnet. The C-magnet has a core which, in cross-section, forms anincomplete annulus thus providing spaced apart pole pieces that define afield gap through which the beam passes. The exciting coil is generallysplit into two sections each encircling a sepa rate one of two polepieces. To provide the alternating gradient field configuration ashereinbefore described, the pole faces curve away from each other sothat the field gap varies in height along an axis at right angles to thebeam orbit whereby the desired bowing of the flux lines is established.

The magnet is the largest and most costly component of an accelerator ofthis type. Thus it is highly desirable that magnet design be optimizedfrom the standpoint of Patented July 26, 1966 providing a field ofmaximum strength and required crosssectional area with a minimuminvestment in core and coil material. Advantages to be obtained byimproved magnet design, over and above reduced material costs, areseveral. For a given output energy, the diameter of the accelerator andconsequent real estate investment may be reduced and the problems ofmagnet alignment minimized. As the construction of a 200 b.e.v.accelerator having a diameter of the order of one mile is presentlybeing planned, these cost factors are of over-riding importance in theart.

A serious limitation of prior accelerator magnet designs, of which theabove described C-rnagnet is a representative example, arises fromsaturation effects. As discussed above, the magnet gap is of varyingdepth along an axis at right angles to the beam trajectory and thus thefield is more intense at one side of the beam centerline than at theother. As a consequence the magnet iron reaches saturation at one sideof the beam centerline at a time when the iron closer to the beamcenterline is carrying considerably less than maximum flux. In a typicalalternating gradient C-magnet formed of iron which saturates at a gapfield strength of about 21 kilo- 'gauss this effect limits the maximumfield in the region of the beam centerline to a conservative workingvalue of about 12.5 kilogauss.

A closely related limitation of conventional magnet designs applies tothe proportion of the total cross-sectional area of the magnet gap whichcan be used for transmitting beam. In order to provide alternatinggradient focussing as discussed above, the field in the magnet gap musthave a fairly specific curvature. In conventionalaccelerator magnets,the curvature of the pole faces that bound the gap is fixed to providethe necessary field configuration in the region of the theoretical beamcenterline. However the field configuration becomes progressively lessideal away from the centerline. At some particular lateral distance fromthe centerline, the field ceases to have an adequate configuration andthis defines the outer limit of the usable cross-sectional area of thegap.

Thus considerable advantage may be obtained if the magnet iron in theregion of the beam centerline can be brought closer to saturation and ifthe desired field configuration can be maintained across a largerfraction of the magnet gap. The present invention provides a magnetconstruction which accomplishes both of these objectives. The inventionachieves these results by means of a specialized core and coilarrangement in which portions of the coil extend along each side of thefield gap and form the lateral boundaries thereof. Each of thesuccessive layers of turns which comprise these portions of the coil isformed to have a curvature conforming to the desired magnetic fluxcurvature within the region occupied by the layer of turns. In addition,the cross-sectional configuration of individual turns differ atdiffering regions of the coil in accordance with the particular desiredcur- [rent density for the regions of such turns. In this manner, thecoil is utilized to establish a predetermined optimum field at each sideof the gap. By thus forcing the field to have the desired configurationat the side of the gap, the coil arrangement causes the field to assumethe preferred configuration throughout the gap.

Relative to conventional alternating gradient accelerator magnets, theabove described coil arrangement draws flux from the narrow side of thegap towards the broader side thereof. As a consequence the fieldstrength in the region of the beam centerline may more closely approachthat of the saturated narrow side. Where iron saturating at a gap fieldof about 21 kilogauss is used as hereinbefore described, the field inthe region of the beam centerline may now be about 18 kilogauss.

Accordingly it is an object of this invention to provide a moreefficient magnet structure for use in charged particle accelerators.

It is another object of the invention to provide a charged particleaccelerator magnet providing a stronger field in the beam aperture.

It is another object of the invention to provide a charged particleaccelerator magnet having a broader usable beam aperture.

It is another object of this invention to reduce the initial investmentcosts of a very high energy particle accelerator.

It is another object of the invention to provide an accelerator magnetrequiring substantially less iron and substantially less copper or otherwinding material to produce a specific field intensity at the beamorbit.

It is another object of the invention to provide a magnet constructionpermitting a reduction in the diameter of extremely high energyaccelerators.

It is another object of the invention to reduce the difficulties ofobtaining and maintaining magnet alignment in a high energy particleaccelerator.

It is another object of the invention to provide a particle acceleratormagnet in which maximum current den sity in the windings is confined toa small proportion thereof thereby reducing the costs and difficultiesof powering and cooling the winding.

The invention, together with further objects and advantages thereof,will be better understood by reference to the following specification inconjunction with the accompanying drawing of which:

FIGURE 1 is a plan view of a portion of a high energy charged particleaccelerator showing several sections of the magnet thereof together withcertain of the associated accelerator components,

FIGURE 2 is a cross-section view of one of the magnet sections of theaccelerator of FIGURE 1 taken along line 22 thereof,

FIGURE 3 is an end view of one of the magnet sections of FIGURE 1 takenalong line 3-3 thereof, and

FIGURE 4 is a cross-section view of a second form of accelerator magnetembodying the invention.

Referring now to FIGURE 1 of the drawing, there is shown a section of aparticle accelerator of the alternating gradient proton synchrotronclass, the apparatus having an overall diameter of the order of one milein order to provide for a maximum beam energy in the region of 200b.e.v. The basic structure and principles of operation of an acceleratorof this general type are known to those skilled in the art and thereforewill be herein described only to the extent necessary for anunderstanding of the inclusion of the present invention therein.

Major elements of such an accelerator include a ringshaped vacuumtubulation 11 through which the particle beam, indicated schematicallyby arrow 12, is circulated, a magnet system 13 for constraining the beamto follow the desired orbit within tubulation 11, and a beamaccelerating station 14. To provide for the positioning of additionalcomponents adjacent the beam orbit and to simplify such operations asbeam injection, beam extraction and magnet alignment, the magnet system13 is comprised of a large number of discrete magnet sections some ofwhich may be spaced apart to leave relatively unobstructed straightsections along the beam orbit.

The representative portion of the accelerator illustrated in FIGURE 1includes six magnet sections numbered '16, 17, 18, 1-9, 21 and 22 ofwhich sections 16 and 17 are widely spaced to provide a long straightsection for accelerating station 14 and magnetic beam focussing lenses23, sections 17 and '18 are spaced apart a shorter distance to providefor a vacuum pumping installation 24, sections 18 and 19 are adjacent,sections 19 and 21 are spaced to provide a short straight section inwhich a subsequent beam focussing lens 26 is disposed, and sections 21and 22 are also spaced to form a short straight section for anadditional vacuum pumping installation '27.

The beam focussing lenses 2-3 and 26, which may be of the known typeutilizing the focussing principles described in the Courant et al.reference hereinbefore identified, act upon a charged particle beam in amanner somewhat analogous to the action of an optical lens on a lightbeam and are provided to compensate for inaccuracies in machineconstruction and alignment and to compensate for the effect ofintroducing the field free straight sections along the beam orbit.

Elements of the accelerating station 14 include a cylindrical drift tube28 which is disposed in alignment with tubulation 11 at a long gaptherein and surrounded by an enlarged vacuum housing 29. Drift tube 23is spaced a small distance from the adjacent ends of tubulation 11,forming a pair of accelerating gaps, and a frequency modulated radiofrequency oscillator 30 is coupled to the drift tube to provideexcitation therefor. Provided the frequency is properly matched to theother operating parameters of the accelerator in a manner understoodwith the art, circulating charged particles receive an increment ofenergy increase in crossing the gaps at the ends of the drift tube '28.

Considering now the novel structure of a representative one of themagnet sections 19, and with reference to FIGURE 2 in conjunction withFIGURE 1, a core assembly '31 encircles and defines a uniquely shapedfield gap 32 through which the vacuum tubulation 11 extends, the corebeing formed of high permeability iron. To reduce power losses from eddycurrents, core 31 is comprised of a large number of laminations orstacked plates 33 which may be held together by longitudinalthroughbolts 34.

The inner surfaces of the core assembly 31 which from the upper andlower boundaries of field gap 32 have similar but reversed hyperboliccurvatures so that, in cross-section, the gap is of greater height atone side of the beam orbit than at the other. Techniques for computing aprecise curvature for the upper and lower boundaries of the field gap toachieve an alternating gradient focussing effect, given a specified setof accelerator parameters, are known in the art and thus will not bedescribed herein. However as will hereinafter be discussed, theinvention allows a stronger field at the beam orbit and a gap having agreater proportion of satisfactory field gradient area to be postulatedas a basis for such computations.

An asymmetrically shaped electrical coil 36 is disposed in gap '32 tofunction as the energizing winding for core 31 and to perform thefurther function of establishing an optimum field configuration withinthe gap. Coil 36 consists of a plurality of series connected turns,twelve in this instance, formed of conductor 37 and disposed withopposite sides of the coil extending along the opposite sides of the gap32. The conductor 37 of coil 36 may be of copper and is hollow toprovide a longitudinal passage 38 for coolant.

Coil 36 is shaped to fill the gap 32 except for the portions thereofoccupied by vacuum tubulation 11 and the regions above and below thetribulation. To impart this shape to the coil 36, the portions of thecoil conductor at the broader side of the gap have broadercross-sectional dimensions than the continuations of the same turns atthe opposite narrowed side of the gap. In addition, in this particularembodiment, the turns are arranged in two horizontal layers at thenarrower side of the gap and in four horizontal layers at the oppositebroader side, the number of vertically oriented layers of turns beingcorrespondingly reduced at the broader side. Th6 number of turns and thearrangement thereof into layers may be varied provided the divisionsbetween turns and the overall form of the coil meet the conditionsherein described.

Considering the shape of the coil 36 and the individual turns thereofstill further, the outer surfaces of each portion of the conductor 37are appropriately curved so that the more horizontal surfaces lie alongmagnetic equi-potential surfaces of the predetermined field within gap32 and the more vertical conductor surfaces lie along flux lines of thefield. This configuration inherently causes the outer surfaces of thecoil as a whole to conform to the adjacent surfaces of core 31. Theconfiguration also results in the coil being essentially solid conductorthroughout except for the coolant passage 38 and a thin layer ofinsulation 39 on the outer surfaces of the conductor 37.

The described coil-conductor shape forces the field to approximate thedesired predetermined configuration within the region occupied by thecoil 36. This in turn causes the field in the beam region, hererepresented by typical fiux lines 40, to assume the desiredpredetermined shape inasmuch as fiux at the central region of a magnetic field must adjust itself in accordance with the flux curvature atthe lateral portions of the field. This effect is supported by thevariation in the cross-section of the coil conductor 37 which, in thedescribed form, establishes a non-uniform current density in the coilthat may be empirically determined to obtain the optimum mag netic fieldintensity at the corresponding region of gap 32. Thus, for example, thecoil 36 may have a lesser current density, and therefore generate alesser field intensity, at the broader side of gap 32 than at the narrowside thereof if necessary to satisfy the field requirements foralternating gradient focussing.

Referring now to FIGURE 3 in conjunction with FIG- URE 2, the portionsof the conductor 67 of coil 36 at the ends of the magnet section 19 arecurved to pass around the vacuum tubulation 11 and the two ends 41 ofthe coil conductor project laterally from the magnet for connection to apower supply as will hereinafter be described. To permit a fiow ofcoolant to be circulated through the conductor, insulated inlet andoutlet fittings 42 are disposed at the coil ends and communicate withthe internal passages 38 thereof. Preferably, inlet and outlet fittings42 are disposed at each end of each half turn section of conductor 37 toincrease the total flow of coolant obtainable with a given supplypressure drop.

As shown in FIGURE 2 in particular, the novel magnet structure lendsitself to a simple and convenient method of assembly. In particular, thecore 31 is formed in two sections, of which the smaller section 43 iscomprised of the portion of the core which spans the broader side of gap32. Core section 43 is secured to the remainder of the core by suitablekeys 44 inserted longi tudinally into grooves which extend along theboundaries between the two sections of the core.

Owing to the tapering cross-sectional configuration of the gap 32,emplacement of the smaller core section 43 functions to secure all ofthe principal elements of the magnet in position in a rigid manner. Thusthe core section 43 bears against the broader lateral sunface of coil'36 thereby tending to wedge the coil into the gap 32. To transmit suchforce evenly to the tubulation 11, the spaces between the tubulation,core 31, and the inner surface of coil 36 are filled by non-magneticelements 45 formed of a radiation resistant material such as ceramic.

To effect alternating gradient focussing, the magnetic field gradientmust be reversed at successive segments of the beam orbit, i.e. the fluxwhich transects the orbit must bow towards the center of the acceleratorat certain sections of the orbit and must bow outwardly at certainintervening portions of the orbit. By convention, the outwardly bowingportions of the field are designated by the letter F and the inwardlybowing field portions by the letter D. Field free gaps are designated bythe letter 0. Using this terminology, magnet section 19 as herein shownand described provides a D-field. To provide an F-field at appropriatesegments of the orbit, a substantially identical magnet section is used,the magnet section being turned end to end so that the broader side ofgap 32 is away from the center of the accelerator.

Thus, wit-h reference again to FIGURE 1, magnet sections 16, 19, and 21are arranged as hereinbefore described to provide a D-field and magnetsections 17, 18 and 22 are turned endwise to provide F-fields. Takinginto account the field free gaps previously described, the acceleratorof FIGURE 1 thus has an [FOEDOD field sequence, which sequence isperiodically repeated around the beam orbit. As will be apparent tothose skilledin the art, this is but one example of a suitable fieldsequence.

The magnet sections are connected in series with pulsed power supply 46and, owing to the reversal of the F-magnet sections relative to theD-ma-gnet sections, the conductor 47 which interconnects the coils ofthe magnet sections connects to the F-magnets at the forward endsthereof and to the D-magnets at the rearward ends thereby forming thenon-uniform pattern of connections illustrated in FIGURE 1.lPretferably, additional series connected magnet power supplies areprovided around the circumference of the accelerator, each having avirtual ground connection, so that it is unnecessary to use high voltagefor magnet excitation.

The invention consists essentially of disposing the excitation coiladjacent the sides of the field gap while forming the coil to force thefield into a preferred configuration. As such it is not limited to theexact'structure described above but may also be applied to differingmagnet configurations. FIGURE 4, for example, illustrates an alternateembodiment which more closely resembles a conventional H-magnet butwhich also directly utilizes the windings to shape the field.

In the embodiment of FIGURE 4, as in the previous instance, a core 48encircles a vacuum tubulation 49 which in turn encloses the beam orbit,the core being formed with :a removable side section 51 held in place bykeys 52. In contrast to the first embodiment of the invention, salientupper and lower pole pieces 53 and 54 respectively are formed on thecore 48 and project towards the tubulation 49. The surfaces 56 of thepole pieces that are adjacent to tubulation 49 are formed with thehyperbolic curvature hereinbefore described.

This configuration provides relatively larger spaces 57 at the sides ofthe field gap 58 in which relatively more coil conductor 59 may bedisposed. Thus, relative to the previous embodiment, lower currentdensity and thus less cooling capacity is required thereby reducingoperating costs. However somewhat more copper and magnet iron isrequired so that the initial costs of the accelerator are higher.

As in the first described embodiment, the conductor 59 which forms thecoil 61 is of non-uniform crosssection with a first pair of outersurfaces formed to lie along magnetic equi-potential surfaces of thefield and with the transecting pair of outer conductor surfaces curvedto follow the flux lines of the field. The innermost portions of thecoil 61 at each side of the field gap 58 extend a small distance betweenpole pieces 53 and 54. The coil thus directly influences the shape ofthe field in gap 58 in a manner similar to that hereinbefore described.

It will be apparent that still other variations are possible within thespirit and scope of the invention and thus it is not intended to limitthe invention except as defined in the following claims.

What is claimed is:

1. In a magnet for guiding a charged particle beam within a particleaccelerator, the combination comprising a ferromagnetic core havingspaced apart pole pieces forming a gap through which said beam passes,said gap having a differing depth at opposite sides of the beamtrajectory whereby the magnetic flux between said pole pieces is bowedwith respect to said beam trajectory, and an excitation coil havingsections extending along opposite sides of said gap, said sections beingof differing height corresponding to said difference in depth ofopposite sides of said gap and having surfaces which are bowed incorrespondence with the curvature of said magnetic flux in the region ofsaid surfaces.

2. A magnet for a charged particle accelerator as described in claim 1wherein said coil is comprised of a plurality of turns and wherein saidturns are of greater cross-sectional area at the high side of said gaprelative to the narrow side thereof.

3. A magnet for a charged particle accelerator as described in claim 1wherein the surfaces of said coil sections adjacent said pole faces havea curvature corresponding to that of said pole faces.

4. In a charged particle accelerator, a beam guiding magnet sectorcomprising, in combination, a ferromagnetic core having a beam passagetherethrough with opposed surfaces of said passage forming pole faces,and an excitation coil for said core having sections extending along thesides of said passage between said pole faces, said coil sections beingformed of a plurality of turns of conductor with opposite surfaces ofthe turns of conductor having curvatures conforming to predeterminedcurvatures for magnetic flux in the region of said surfaces.

5. A magnet for :a charged particle accelerator comprising aferromagnetic core element transpierced by a particle beam passagehaving opposed pole face surfaces of substantially hyperboliccross-section whereby said passage is of greater height at a first sideof the beam trajectory than at the opposite side thereof, and anasymmetric winding for said core having sections extending alongopposite sides of said passage and extending between said pole facesurfaces whereby said winding is of greater height at said first side ofthe beam trajectory than at the opposite side thereof, said windingsections having inner surfaces which are curved in conformity with thecurvature of the adjacent portions of the magnetic field within saidpassage.

6. A magnet for a charged particle accelerator as described in claim 5wherein said winding is comprised of a plurality of turns of conductorand wherein the divisions between uccessively more lateral layers ofsaid turns have progressively changing curvatures in order to lie alongthe flux lines of said magnetic field.

7. In a charged particle accelerator of the class having an evacuatedcurvilinear tubulation through which the particle beam orbit passes, amagnet structure comprising a ferromagnetic core enclosing said vacuumtubulation and having pole face surfaces at opposite sides thereof forestablishing a magnetic field therethrough, said pole face surfacesbeing divergent with respect to the plane defined by said particle beamorbit whereby said magnetic field has a gradient in the region of saidvacuum tubulation, an asymmetrically shaped winding having sectionsextending within said core along opposite sides of said vacuumtubulation with each section spanning the gap between said pole faces,the surfaces of said winding sections which are adjacent said pole facesbeing shaped to conform thereto and the inner surfaces of said windingsections which face said vacuum tubulation being shaped to follow apredetermined curvature for said magnetic field in the region of saidinner surfaces, and a pulsed direct current power supply coupled to saidwinding.

8. A charged particle accelerator magnet structure as described in claim7 wherein said winding is comprised of a plurality of turns of conductorand wherein said conductor has varying cross-sectional dimensions atdiffering regions of said winding to establish a predetermined currentdensity distribution within the region occupied by said winding.

9: A charged particle accelerator magnet structure as described in claim7 wherein said winding is comprised of a plurality of turns ofconductor'which has changed dimensions at successive segments thereof,the conductor having a first pair of opposite lateral surfaces with acurvature conforming to that of the adjacent magnetic field and having asecond pair of opposite lateral surfaces curved to follow magneticequi-potential surfaces of said field.

10. A charged particle accelerator magnet structure as described inclaim 7 wherein said ferromagnetic core is formed of two discreteelements, a first of said elements being comprised of the portion of thecore which extends between said pole faces at the more widely spacedside thereof and comprising the further combination of clamping meanssecuring said elements together as an integral unit.

11. A charged particle accelerator -magnet structure as described inclaim 10 comprising the further combination of spacer material fillingthe regions between said vacuum tubulation and said winding sectionswhereby said first core element and said clamping means may be utilizedto maintain a wedging pressure on said winding to bind the core andwinding and vacuum tubulation into a rigid unit.

References Cited by the Examiner UNITED STATES PATENTS 12/1953 Wideroe317200 X 4/1959 Courant et al. 328235

1. IN A MAGNET FOR GUIDING A CHARGED PARTICLE BEAM WITHIN A PARTICLES ACCELERATOR, THE COMBINATION COMPRISING A FERROMAGNETIC CORE HAVING SPACED APART HOLE PIECES FORMING A GAP THROUGH WHICH SAID BEAM PASSES, SAID GAP HAVING A DIFFERING DEPTH AT OPPOSITE SIDES OF THE BEAM TRAJECTORY WHEREBY THE MAGNETIC FLUX BETWEEN SAID POLE PIECES IS BOWED WITH RESPECT TO SAID BEAM TRAJECTORY, AND AN EXCITATION COIL HAVING SECTIONS EXTENDING ALONG OPPOSITE SIDES OF SAID GAP, SAID SECTIONS BEING OF DIFFERING HEIGHT CORRESPONDING TO SAID DIFFERENCE IN DEPTH OF OP- 